It's not necessarily a fact that small families are the best, but the fact that raising just one child is extremely expensive. There is the hospital bill , one must have good medical/dental (very expensive) for those toothaches , possible braces, fevers, scrapes, childhood diseases, not to mention the many shots children get during their younger years. There is clothing, food, a good home to live in, education and other hidden costs. Your child is better adjusted if they (a boy) are put into a sport at the age of 5 or 6, or if it's a girl , into dancing, etc., to keeping them off the streets and also give them insight as to what the future can hold for them. It costs many thousands of dollars to raise one child, and I use to know the stats on it, but it has since changed .
If a family can afford more than 2 children (good to have sibling) then they should go for it. I have known families during the 50s to 70s where there were 5 - 8 kids in a family and they were all well adjusted kids, not to mention the oldest child always ends up helping mom and pop out. Large families can be fun , but if you can't afford it this can cause a great deal of pressure on the parents and either shorten their life span or end in divorce. To have a large family you have to be well organized